Re: [PATCH V7 v7] virtio: introduce SUSPEND bit in device status

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 03, 2024 at 10:09:34AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, September 3, 2024 3:16 PM
> > 
> > On Tue, Sep 03, 2024 at 05:05:40PM +0800, Zhu Lingshan wrote:
> > > >> Are there any similar
> > > >> constraining for other states like RESET or DRIVER_OK? Don't assume
> > > >> any other states transitions are faster than SUSPEND.
> > > > DRIVER_OK does not suffer from it because it is async notification.
> > > > A device may start slow after DRIVER_OK.
> > > > SUSPEND operation cannot rely on such async behavior.
> > > The driver can set suspend and re-read & wait. This is a common routine in
> > the driver.
> > 
> > I don't buy all this talk about special machinery, some 10 nor gates, the cost
> > is in the noise - we are talking about cards with several ARM processors on
> > chip.
> They may not respond back under 50nsec.
> How many cores would you like to dedicate and for how many PCI functions?
> And how they can be easily underutilized...
> So, it is not a noise.
> 
> Circuity is the device abstraction that we keep instead of digging down on gates.
> 
> Also can you please explain how can it work without VM_EXIT? You have better insight to this than me..

I don't know which interface we are arguing about here, when there's
a specific proposal, I can comment.

> > 
> > But I don't like it that looking at the registers, one does not know the device
> > state. Hidden state is bad for debuggability.
> > We have 4 states:
> > suspending->suspended->resuming->resumed
> > so we need a register with at least 2 bits.
> > 
> > we could steal 2 bits from status but it seems a bit much.
> >
> This is why letting the status tell the status and control register to control thing is elegant.


No argument here.